Ramp generator circuits are well known in the art. It is a common technique in a ramp generator circuit to provide current sources to alternately charge and discharge a capacitor. The voltage on the capacitor responds to the alternating charge and discharge operation by rising and falling, respectively, with a ramp shape (producing a sawtooth or triangle waveform). However, because of a number of factors including the effects of process, voltage, aging and temperature, it has proven to be quite difficult to control the operation of the ramp generator circuit so that the capacitor voltage rises and falls with a constant and accurate slope over the operating life of the circuit. A number of circuit applications, including for example pulse width modulation, rely on the ramp output of the ramp generator circuit and cannot tolerate inaccuracy in the ramp slope.
There is accordingly a need in the art for a ramp generator circuit having an accurate and constant slope.